UCN Source Research Articles

Very cold and ultra cold neutron sources for ESS

The goal of the “Workshop on Very Cold and Ultra Cold Neutron Sources for ESS” was to discuss scientific cases, ideas and possibilities for the implementation of sources of Very Cold and Ultra Cold neutrons at the European Spallation Source. The ESS facility, presently under construction, offers several possibilities for in-pile UCN or VCN sources, in primis thanks to the available space below the spallation target where additional neutron sources can be placed to complement those above the target. Neutron beams can be extracted over a wide angular range with a grid of forty-two beamports with 6° average angular separation, allowing future instruments to be installed which may view either the upper or lower moderator systems. Of greatest interest for fundamental physics is the so-called Large Beamport foreseen for the NNBAR experiment. This beamport is also particularly well suited to feed a UCN source, for which several ideas were presented that employ either superfluid helium or solid deuterium as established neutron converter materials. Concepts for VCN sources make use of novel materials for VCN production and/or advanced reflectors to increase yields in the coldest part of the neutron spectrum from a cryogenic neutron source. In this paper we discuss these ideas and the possible locations of UCN and VCN sources at ESS.

Growing solid deuterium for UCN production

We have experimentally studied growing a large (about 1 liter) ortho-deuterium crystal in a real UCN source cryostat and recorded the growing process optically using a camera. The best quality was observed when growing the crystal directly from a vapor phase. The crystal was grown at different mass flows of deuterium and annealed at different temperatures. Optimum conditions were found for both, obtaining an optically transparent crystal and cooling it down with minimal damage. We found that the quality, final shape and changes during annealing of the crystal are very much dependent on the temperature profile of the cryostat walls.

On the Possibility of Creating a UCN Source at a Periodic Pulsed Reactor

— The possibility of creating a UCN source at a periodic pulsed reactor is considered. It is shown that the implementation of the principle of time focusing based on nonstationary neutron diffraction and the idea of pulse filling of the UCN trap allow one to create a sufficiently intense UCN source at a periodic pulsed reactor.

Status of UCN supersource at WWR-M reactor

The WWR-M reactor at NRC «Kurchatov Institute» - PNPI is going to be equipped with high-density ultracold neutron source. Method of UCN production is based on their accumulation in the superfluid helium at 1.2 K temperature. Thus, the source will provide the UCN density at EDM spectrometer equals to ρ = 1.3⋅104 cm−3 which is 2 order magnitude greater than the output density of existing UCN source in the world. An extensive program of fundamental researches such as measuring of neutron lifetime and searching of neutron-antineutron oscillation is planned. In addition, CN and VCN beams are going to be equipped with condensed matter physics experimental setups. The design of the UCN source has been completed, complex tests at full-scale model showed that is possible to maintain superfluid helium under reactor heat load; calculations of an UCN source passive shielding, which ensures source safe operation, is completed. At the moment the process of UCN source manufacturing is taking place.

Нейтроноводная система ультрахолодных и холодных нейтронов на реакторе ВВР-М

AbstractThe results of calculation of fluxes of ultracold (UCNs), very cold, and cold neutrons at the output of neutron guides of the UCN source with superfluid helium at the WWR-M reactor are presented. UCN density ρ_35L = 1.3 × 10^4 n/cm^3 in the trap of the electric dipole moment (EDM) spectrometer was obtained by optimizing source parameters. This UCN density in the EDM spectrometer is two orders of magnitude higher than the UCN density at the output of the available UCN sources. The flux density of cold neutrons with a wavelength of 2–20 Å at the output of a neutron guide with a cross section of 30 × 200 mm^2 should be as high as 1.1 × 10^8 n/(cm^2 s), while the flux density of very cold neutrons (50–100 Å) at the output of the same neutron guide should be 2.3 × 10^5 n/(cm^2 s). An extensive program of fundamental and applied physical research was mapped out for this source.

The n2EDM experiment at the Paul Scherrer Institute

We present the new spectrometer for the neutron electric dipole moment (nEDM) search at the Paul Scherrer Institute (PSI), called n2EDM. The setup is at room temperature in vacuum using ultracold neutrons. n2EDM features a large UCN double storage chamber design with neutron transport adapted to the PSI UCN source. The design builds on experience gained from the previous apparatus operated at PSI until 2017. An order of magnitude increase in sensitivity is calculated for the new baseline setup based on scalable results from the previous apparatus, and the UCN source performance achieved in 2016.

Status of UCN Source at WWR-M Reactor

The WWR-M reactor at PNPI is going to be equipped with an ultacold neutron source of high density. Method of UCN production is based on their accumulation in the super fluid helium due to particular qualities of that quantum liquid. The possibility of maintaining the temperature T = 1.371K with a thermal load of P = 60W was shown experimentally, while the theoretical load is expected to be P=30W. The project envisages the installation on UCN beams the experimental setups of various research projects such as searching for the nEDM, measure the neutron lifetime, and the observation of neutron to antineutron oscillation. In addition to UCN beams, three beams of cold and verycold neutrons are planned. Six experimental setups will be installed on these beams. At present, a vacuum container of the UCN source has been manufactured and the manufacture of low-temperature deuterium and helium parts of the source has been started.

UCN source with superfluid helium at WWR-M reactor

The WWR-M reactor at PNPI is going to be equipped with an ultracold neutron source of high density. Method of UCN production is based on their accumulation in the super fluid helium due to particular qualities of that quantum liquid. The satisfying storage time of UCN at WWR-M reactor in the super fluid helium exists at a temperature below 1.2 K. Our source aims at obtaining a density of UCN equals to 104 n/cm3, two orders of magnitude exceeding that in existing sources presently available in the world. Increase in the density of UCN will raise the accuracy of the measurement of the neutron electric dipole moment (EDM) of an order of magnitude, which is fundamentally important for the problem of CP violation. The most intense sources of UCN allows PNPI become the centre of fundamental researches with ultracold neutrons.

Spallation UCN Production for nEDM

Abstract A new superthermal UCN production in He-II, which is placed in a spallation neutron source, is discussed. In the new UCN source, the production rate is expected to be 200 UCN/cm 3 /s at a proton beam power of 500 MeV×40 μA and the UCN maximum energy of E c = 210 neV. The γ heating in the He-II can be removed by means of 3 He pumping according to calculations. For an EDM measurement, UCN are extracted from a He-II bottle through an aluminum window by using a superconducting magnet. The possibility of a 129 Xe magnetometer for the EDM measurement is also discussed.

Towards a new measurement of the neutron electric dipole moment

Precision measurements of particle electric dipole moments (EDMs) provide extremely sensitive means to search for non-standard mechanisms of T (or CP) violation. For the neutron EDM, the upper limit has been reduced by eight orders of magnitude in 50 years thereby excluding several CP violation scenarios. We report here on a new effort aiming at improving the neutron EDM limit by two orders of magnitude, down to a level of 3 × 10−28 e·cm. The two central elements of the approach are the use of the higher densities which will be available at the new dedicated spallation UCN source at the Paul Scherrer Institute, and the optimization of the in-vacuum Ramsey resonance technique, with storage chambers at room temperature, to reach new limits of sensitivity.

The spallation target of the ultra-cold neutron source UCN at PSI

A new type of ultra-cold neutron source, based on the spallation process, is scheduled to start operation at PSI in 2008. The UCN source will be operated in a pulsed mode using the 1.2 MW (590 MeV, 2 mA) proton beam delivered from the ring cyclotron. In this paper we discuss the neutronic and the thermo-mechanical behavior of the target assembly under pulsed operation.

Activation calculations for the target of a spallation ultra-cold neutron source at PSI

A spallation ultra-cold neutron source--UCN source--is scheduled to start operation at PSI in 2006 using up to 2 mA 590 MeV protons from the ring cyclotron. It will be operated in a pulsed mode with an average current of 20 microA. For safe maintenance, during operation as well as handling, transport and storage of the UCN target assembly after its lifespan, detailed knowledge about the activation induced by the impinging protons and secondary radiation fields is required. The Monte Carlo transport code MCNPX was coupled with the European Activation System--EASY--to calculate the residual nuclide production in the UCN target assembly. The nuclide inventory is finally used to design the shielded exchange flask that is needed to safely remove and transport the UCN target assembly after its lifespan to a hotcell for dismantling.

Studies of a solid-deuterium source for ultra-cold neutrons

The results of experiments and calculations for a solid-deuterium UCN source are presented. It was experimentally shown that the UCN yield is increased by a factor 3 at the solidification of liquid deuterium and a factor 10 at a temperature 10 K, with respect to the liquid phase. The gain factor for UCN yield with respect to hydrogen gas or deuterium gas at room temperature is equal to 1230.

Spectroscopic study of scattering of UCN by liquid HE 4

We report on our first recent measurements of the energy spectrum of upscattered Ultra-Cold-Neutrons in liquid He4 at temperatures around 1 Kelvin. The measurements were made on the Superthermal UCN Source at the ILL [1]. This source produces UCN's in liquid He 4 by down-scattering (phonon-emission) of 10 Å neutrons. The UCN's produced in this way are stored in the He 4 container, since they are totally reflected from the walls. The UCN's can gain energy through scattering by Phonons [3] and Rotons [3], and hence leave the container. We have measured the spectrum of these upscattered neutrons by using a pseudo statistical chopper [4]. The first preliminary measurements show a discrepancy with the spectrum expected on the basis of phonon and roton scattering.

Operation of a superthermal ultra-cold neutron source and the storage of ultra-cold neutrons in superfluid Helium4

We report the first successful operation of a superthermal UCN source and the observation of the storage of Ultra-Cold Neutrons in superfluid Helium4 at Temperatures in the range 0.5–1.15K. From the observations we obtain the total cross section for UCN scattering by superfluid Helium. The temperature dependence of the total cross section is found to be consistent with calculations which include the effects of phonon absorption and phonon and roton scattering.